The present invention relates to a fuel assembly loaded in a boiling water reactor and a reactor core containing such a fuel assembly, and more particularly to a fuel assembly and a reactor core which contribute to achievement of higher burn-up and an improvement in fuel economy.
In existent boiling water reactors, a fuel assembly is built up in such a manner that a fuel rod group (fuel bundle) is constituted by a number of fuel rods arrayed in a square lattice pattern and containing nuclear fuel material and at least one moderator rod containing no nuclear fuel materials, and a channel box is arranged to surround the fuel rod group. The presence of the channel box defines a coolant path passing the fuel rod group inside the channel box and a coolant path outside the channel box. Light water as a coolant and a moderator flows through those paths. The light water flowing through the inside of the channel box boils, but the light water flowing through the outside of the channel box does not boil. Accordingly, the fuel rods making up the fuel assembly are subjected to different profiles of moderator distribution depending on whether they are located near to or far from the channel box.
Recently, raising burn-up of a fuel assembly has been attempted from the standpoints of effectively utilizing uranium resource and reducing the amount of spent fuel generated. For this purpose, it is required to increase mean fuel enrichment of a fuel assembly. With fuel enrichment of fuel rods increasing, however, a number density of fuel material is increased, hence a fission cross section is enlarged. At corners of a channel box including a fuel assembly, because neutrons produced by nuclear fission are effectively moderated with light water, that flows outside the channel box surrounding fuel rods, to become thermal neutron flux, the thermal neutron flux is intensified around the fuel rods near the channel box corners. As a combined result of the above effects, those fuel rods which are arranged at the corners of the channel box including the fuel assembly exhibit higher power and a higher local power peaking factor so that a thermal design margin decreases.
As one prior art for solving the above-mentioned problem, it has been proposed to make an outer diameter of the fuel rods arranged at the channel box corners smaller than that of other fuel rods, thereby suppressing an increase in the fission cross section due to greater enrichment, lowering the power per unit length of the fuel rods arranged at the channel box corners, and reducing their local power peaking factor (see JP, A,52-9792 and JP, A, 58-129385, for example: this prior art will be hereinafter referred to as the first prior art). As another prior art, it has also been proposed to provide a central region where a plurality of fuel rods are arranged in a square lattice pattern and a peripheral region in which a plurality of fuel rods are arranged in a triangular lattice pattern and which surrounds the central region, and to increase a diameter of some fuel rods correspondingly to an increase in distance between those fuel rods and the channel box resulted from the presence of the triangular lattice pattern, thereby increasing the power of those fuel rods, lowering a share of the power to be borne by the fuel rods arranged at the channel box corners, and reducing their local power peaking factor (see JP, A,1-308994: this prior art will be hereinafter referred to as the second prior art).